1. Field of the Invention
The present invention relates to: a Fresnel lens sheet for solar concentration which has a function to concentrate direct light from the sun and is used in a solar power plant to concentrate sunlight on solar cell units in attempt to improve the power generation efficiency; and a design method therefor.
2. Background Art
A concentrator photovoltaic device is a device configured to generate electric power by concentrating sunlight on solar cells through concentrating lenses. Each solar cell only needs to have an area large enough to receive the light concentrated by its corresponding lens, and thus can be reduced to a size that is significantly smaller than the lens size. Accordingly, the amount of the solar cell, which is very expensive among other parts of the power generating device, can be reduced, allowing a cost reduction. On the other hand, the use of lens requires the device to be used in an area having a large amount of direct light (long sunshine duration). In view of this, the concentrator photovoltaic device has started to be widely used as a power supply in areas having long sunshine durations and large lands that allow the device to be built in a large scale. Japanese Patent Application Publication No. 2006-343435, for example, can be cited as an example of the concentrator photovoltaic device.
To utilize light highly efficiently in general, a highly transparent acrylic resin or polycarbonate resin or the like is used for lenses used in general optical applications. These resins generally contain a predetermined amount of an ultraviolet absorber to prevent coloration, resin degradation, and the like of the lenses due to the influence of ultraviolet rays. These optical members are generally used indoors as a screen and a backlight member of a television, and therefore the amount of ultraviolet rays directly applied thereto is low, and the concentration of the added ultraviolet absorber is low as well. Moreover, as shown in an embodiment, etc. of Japanese Patent Application Publication No. Hei 05-156113, it is possible to add a large amount of the ultraviolet absorber to improve the light resistance of the resin. However, coloration may be caused by the ultraviolet absorber, which in turn causes optical problems such as decreasing the transmittance and external problems such as yellowing, as well as problems such as decreasing the formability. For this reason, the ultraviolet absorber is generally added at the lowest possible concentration.
However, due to the facts that lenses for solar concentration are used in an environment where they are irradiated with a significantly large amount of ultraviolet rays, they are required to be used for a long term such as 20 years or longer, and so on, it is necessary to add a large amount of ultraviolet absorber as described above, but at the present a proper amount thereof concerning influences of the coloration and degradation on the decrease in transmittance, an influence on the formability, and the like is unknown.